Object detection system



Feb. 22, 1966 G. BOJKO 3,237,191

OBJECT DETECTION SYSTEM Filed May 28, 1963 E 01 I 60 8a jg AUDIO LOW PASS AMP. FILTER INDICATOR I 70 i I re 6 o Invezzioa: Geodge Bo ido,

United States Patent Office Patented Feb. 22, 1966 3,237,191 OBJECT DETECTION SYSTEM George Bojko, Framingham, Mass., assignor to Pinkertons, Inc. Filed May 28, 1963, Ser. No. 283,760 10 Claims. (Cl. 343-5) This invention relates to intrusion alarms, and more particularly to a signal generator and antenna system for use in intrusion alarms.

Various types of intrusion alarms have been developed in the past. The operation of a vast majority of these hitherto known systems is based on the phenomenon called Doppler effect wherein motion of an object toward or away from an antenna causes the energy reflected from the object to arrive back at the antenna at a different frequency than the transmitted wave. When these two frequencies are combined, a resultant beat frequency is produced which is called the Doppler shift. The advantage of employing Doppler in the usual system is that a frequency is being detected qualitatively rather than quantitatively. This means that it can be filtered and amplified, and as a result a high degree of sensitivity is possible. Intrusion alarm systems, however, based on the Doppler principle are otherwise vulnerable for various reasons. First, an object moving on a circle the center of which is at the antenna is difficult, if not impossible, to detect. Second, any spurious transmissions of short wave radio transmitters from police cars, for instance, can cause false alarms. In fact, moving curtains can set up a Doppler shift, and if the system is sensitive enough tobe effective against persons, a curtain may well ring the alarm. Another source of false alarms in Doppler systems is moving objects of high reflectivity outside the normal range of the system. Thus, if a large metal bus goes by, even at a distance, enough reflection may result to compare equally, at the antenna, with the relatively low intensity reflection of a person near the antenna.

One system which avoids a majority of the drawbacks of the Doppler type alarm is described in United States Patent to Reynold S. Chapin, No. Re25,100. In the Chapin patent a standing wave pattern is established by the radiated energy and the alarm is excited at least in part by detecting a change in the standing wave pattern caused by the presence of an intruder in the area of the pattern. This latter type of detection is distinguishable from the conventional Doppler system in that a major share of the reflected energy returns to the antenna at the same frequency as the incident radiation. Thus detection largely results, not qualitatively from beating a reflected wave against a transmitted wave but rather from a quantitative shift in the antenna impedance due to a shift in the positions of the nodes and anti-nodes in the standing wave pattern due to change of position of the intruder in the field.

One drawback, however, of the standing Wave type device, described in the Chapin Reissue Patent No. 25,100 is that the system in that patent is still quite sensitive to Doppler in addition to standing wave changes. Thus, while a person walking around the antenna of that device cannot escape detection because when he does so he also interferes with the standing wave pattern, the system additionally responds to ordinary Doppler shift frequencies reflected directly from moving objects approaching or receding, or produced by nearby transmitters. Thus, police car or amateur two-way radio transmitters and large reflectors such as trucks and busses have tended to cause false alarms with that system. Also, although small moving objects such as dogs, cats and mice and even thin objects such as curtains were not as likely to cause false alarms with that system, as with purely Doppler systems, because they did not materially disrupt the standing wave pattern, they still could, at times, cause false alarms. Also line voltage fluctuations or other spurious voltage changes inherent in the system cause false alarms.

An object of this invention, therefore, is to provide an intrusion alarm system which, at one and the same, (a) enjoys the full benefits of the standing wave pattern mode of operation of the Chapin Reissue Patent No. Re25,l00 but with increased sensitivity; (b) eliminates false alarms due to line voltage fluctuations or other fluctuations inherent in the system, (c) virtually eliminates all false alarms from energy received, from nearby transmitters, and (d) greatly reduces Doppler shift type response even in the detection area.

In the accomplishment of these and other objects of my invention in a preferred embodiment thereof, I employ a low energy micro-wave CW detection system operating in the 400 megacycle range. The circuitry for signal generation and detection is similar to that shown in the Chapin Reissue Patent No. Re25,l00. However, instead of employing a single antenna, I use a pair or a plurality of pairs of spaced antennas, and a balancing circuit by means of which all equal changes in impedance at each antenna are cancelled out. In addition I employ a tuning circuit in each antenna. The tuning circuit eliminates the police car type of transmission and the balancing circuit cancels out spurious fluctuations in voltage or fluctuations due to inherent imperfections in the system. Doppler shift type frequencies also are eliminated completely for objects moving on a line normal to and bisecting the line between the two antennas, and for all other motion in the field, the Doppler response is more or less minimized. In theory, there is one point at which the Doppler type response of my system can be equal to that of the Chapin system provided the two antennas are spaced apart precisely an even multiple of the wave length, but this condition is easily avoided, and for all other positions the Doppler type response is substantially minimized.

In addition, in my system, the changes in transmittedwave frequency caused by changes in antenna impedance (which in turn influence the oscillator), additionally augment the distortion of the standing wave pattern caused initially by a change of position of the object being detected. This leads to increased sensitivity in my system.

The standing wave pattern, on the other hand, is heterogeneous in nature because it is formed by the addition or subtraction of a large number of superimposed reflections all oscillating at the frequency of the transmitted wave. Therefore, the nodes and anti-nodes of the standing wave pattern are spaced in a non-uniform manner and changes in their locations take place non-uniformly as an object in the field changes position. The result is that the impedances at the two antennas do not change uniformly, and cancellation does not occur.

By this technique, it is, therefore, a feature of my invention that much more emphasis is placed on the standing wave pattern concept first taught but not fully exploited by the device described in the Chapin Reissue Patent No. Re. 25,100. In fact, by a careful analysis of the primary reflection pattern with my invention, it will be seen that a far more effective and intense pattern can be created by one or more pairs of antennas equipped with my cancelling or balancing circuits. In explaining the standing wave system employed in my invention, the primary or one bounce reflections need only be considered. Usually only a small percentage of the transmitted energy is reflected from conventional objects in dwellings, such as tables, chairs, Wall's, floors, ceilings, persons, etc. I calculate that an average reflection intensity of somewhere around 310% of the energy striking any one point in a room is typical. Of course, wide variations in this, also occur depending upon the materials involved, and the secondary reflections can definitely have an influence in the system.

When an object is placed in a room, wherein the intrusion alarm of my invention is located, there will be primary reflections from the object to both antennas from transmissions received from each. Thus, a theoretical total of four primary reflections are available for frequency shift (Doppler) purposes due to object motion, but since the balancing circuit will normally at least partially cancel the response due to these Doppler type reflections, and since the theoretical maximum Doppler can occur only when the antennas are spaced apart by an even multiple of the wave length and the object is approaching both antennas along an extension of the line between them, the

- detection of Doppler type energy with my system is greatly reduced. However, since the reflections which create the standing Wave pattern with my system are greatly increased by the two antenna arrangement, an increase in sensitivity results. Of course, in a more complex reflection environment, such as one finds in any conventional room, the number of available standing Wave reflections increases While the Doppler type primary reflections remain the same, and accordingly the above-stated emphasis on standing wave type response will normally be substantially greater.

A further feature of my invention arising out of its increased emphasis on the standing wave type mode of operation is that it is much less sensitive to motion from small objects, such as mice, cats, dogs, birds, etc., because they do not have enough mass to cause significant distortion of the standing wave pattern and their Doppler type disturbance is largely eliminated by the balancing circuit.

Another feature of my invention relates to the spacing and positioning of the antennas. They must be far enough apart and placed so that the heterogeneous nature of the standing wave pattern results in non-uniform responses at the respective antennas when an object in the field changes position. Ordinarily a spacing of several wave lengths is suflicient, but greater distances are feasible. Even at fifty feet the transmissions of the respective antennas augment the standing wave field surrounding the other. I prefer a spacing of between about 15 to 30'.

A further feature of my invention intended to maximize standing wave values and minimize Doppler values is the use of reflectors in the field. By placing highly reflective objects in the standing wave field, I can greatly increase the primary wave reflection pattern intensity. This would not be useful without my balancing circuit, but with it the Doppler components involved in this increased field intensity are laregly eliminated, While the standing wave components are augmented.

These and other objects and features of the invention will be apparent from the following Written description and drawings, in which:

FIG. 1 is a diagram of the detection system showing the oscillation generator in schematic form and the other components of the system in block diagram;

FIG. 2 is a view in perspective of an inverted antenna assembly showing the antenna, antenna base and associated electrical and structural components;

FIG. 3 is a schematic drawing of the antenna and associated electrical components; and,

FIG. 4 is a schematic drawing of the antenna and associated electrical components showing a modification therein.

Turning now to the drawings, FIG. 1 shows the oscillation generator indicated generally at 2 in a schematic representation while the other major components of the object detector system are shown in block diagram. The oscillator 2 generates a continuous sinusoidal wave of radio frequency energy which is radiated from two antenna assemblies 4 and 6 to create a standing wave pattern of alternate points of maximum and minimum energy located generally in the path of a moving object. The radio frequency energy reflected from the object and the surroundings intercepts antenna assemblies 4 and 6 and produces a voltage across the impedance of each antenna thereby causing current to flow through the impedances. The returned energy from the object and surroundings combines with the oscillator energy in the impedance of each antenna to form a new voltage therein which is the vector sum of the transmitted and received voltages. The resultant voltage from each antenna system will change in amplitude and phase in accordance with the position of the object in the radiation pattern and this change provides a means for detecting the presence or change of position of the object. The two resultant voltages are combined in a manner to be explained subsequently and the combined voltage representing the level of intensity of the standing radio frequency field pattern is coupled through capacitor 8 to an audio amplifier 10.

The amplified audio signal is fed through a low pass filter system 12 to an indicator 14 which provides a visual, aural or electrical indication of the change in the standing wave pattern established by the radiated radio frequency energy caused by change of position of an object therein. 7

The oscillation generator 2, shown schematically in FIG. 1, is representative of the various circuits which will produce and sustain oscillations in the very high frequency or ultra high frequency spectrum. Although the frequency of operation for the object detector is not critical, it has been found that the meter portion of the UHF band is well suited for detecting the presence of a person because the wave length at 400 megacycles creates a standing wave pattern in which the equi-potential points are separated by a distance roughly comparable to the physical size of the object to be detected. This standing wave pattern produces an extremely sensitive system because only a small change in the position of the detected object will cause a large phase shift in the standing wave pattern which results in a correspondingly large change in the impedance of the antenna.

The oscillation or signal generator 2 consists of a selfexcited, twin triode electron tube oscillator 16 and associated components. The electron tube 16 can be any one of the twin triode series of tubes whose interelectrode capacitance is sufficient to provide signal feedback of the proper phase and amplitude to sustain oscillation. The oscillator tube 16 is connected in a grounded cathode configuration with the grid bias being provided by the grid leak current flow through a grid resistor 18. The output of the oscillator tube 16 is developed across a tuned plate tank circuit consisting of inductor 20 and capacitor 22. The plate inductor 20 is formed from a length of No. 11 A.W.G. wire bent into an elongated U shape and connected to the plates of the twin triode 16. The plate tank circuit is tuned by varying the capacitance of the tank capacitor 22. This is accomplished physically by changing the separation distance between the fiat tuning disc of the capacitor 22 and the plate line inductance 20.

The plate supply voltage for the plates of the twin triode 16 is applied to the electrical center of the U shaped plate line inductor 20 through a decoupling circuit consisting of a radio frequency choke 24 and a shunt capacitor 26.

The radio frequency output of the oscillator tube 16 is coupled to the antenna circuit by the mutual inductance of the plate line inductor 20 and a link coupling inductor 28. The link inductor 28 is formed in the same manner as the plate line inductor 20 and mounted in a plane parallel to the plane of the plate line inductor 20 at a distance determined by the amount of the inductive coupling desired between the respective inductors.

The induced radio frequency energy in the link inductor 28 is coupled to the dual antenna assemblies 4 and 6,

through identical circuits formed by capacitors 30 and 32, jacks 34 and 36 and the antenna feed lines 38 and 40. Radio frequency chokes 42 and 44 block the RF energy from the signal balancing circuit consisting of resistors 46 and 48, potentiometers 50 and 52 and resistors 54 and 56 whose functions will be explained subsequently.

Referring now to FIGS. 2, 3 and 4, FIG. 2 is a perspective view of one of the dual antenna assemblies which has been inverted to show more clearly an antenna base 58, a radiator 60 and the associated electrical components 62 mounted within the base 58 on a metal plate 63. The metal plate 63 is physically and electrically connected to the base 58 to provide a common ground for the components 62. The antenna base 58 is formed by spin casting aluminum in the shape of a truncated hemisphere having a diameter of approximately 5 /2 and an altitude of 2". The radiator 60 is a length of aluminum rod approximately 9" in length and /8" in diameter. The base 58 functions as a ground plane for the radiator 60 which is secured to the fiat part of the truncated hemispherical base 58, but electrically insulated therefrom by means of a feed-through porcelain insulator 64, a portion of which can be seen in FIG. 2.

The electrical components 62 mounted within the antenna base 58 can best be understood by comparing the perspective view of FIG. 2 with the schematic circuits shown in FIGS. 3 and 4. With a dual antenna system as shown in FIG. 1, the circuit of FIG. 3 would be employed in one antenna assembly while the circuit of FIG. 4 would be utilized in the other antenna assembly. Although only one pair of antennas is shown, the concepts disclosed are equally applicable to multiple pairs of antennas and hence it is not intended to limit the invention to a single pair of antennas.

The two antenna assemblies 4 and 6 are connected to the oscillator assembly 2 through antenna feed lines 38 and 40, jacks 34 and 36 and capacitors 30 and 32, respectively. The antenna feed line can be any suitable transmission line, however, a coaxial cable would norrnally be employed to connect the two antenna assemblies 4 and 6 to the oscillator assembly 2. The outer conductor or shield of the coaxial cables 38 and 40 is grounded at the oscillator assembly 2 through the shells of jacks 34 and 36, respectively, and at the antenna assemblies 4 and 6 through the shells of corresponding jacks 71 mounted in the antenna bases 58. Referring now to FIGS. 3 and 4, the center conductor of the coaxial feed line is connected to the antenna tank circuit inductor 66 at a point above ground which corresponds to the characteristic impedance of the feed line to provide a matched termination for the transmission line. The cold or ground end of the antenna tank circuit inductor 66 is held at RF ground by capacitor 68.

The antenna tank circuit, consisting of inductor 66 and a variable capacitor 70, is tuned to the operating frequency of the signal generator 2 by adjusting the capacitance of capacitor 70. When the antenna tank circuit is tuned to resonance a maximum amount of the oscillatory radio frequency energy is transferred from the signal generator 2 to the antenna tank circuit and coupled to the radiator 60 through a series capacitor 72. The series capacitance coupling of the antenna tank circuit to the radiating element 60 significantly improves the interference ratio of the detection system and renders the sensitivity of the system relatively independent of antenna loading. The value of capacitor 72 is selected to provide sufficient capacitive reactance to block or greatly attenuate spurious frequencies falling below the desired range of signal frequencies while presenting an effective RF short circuit to the desired signal frequencies. Interference from spurious RF transmission is further reduced by the selective resonance of the high Q, sharply tuned antenna tank circuit. Any spurious RF transmissions which do fall within the narrow frequency range of the tuned antenna circuit will not affect the moving object 6 detection system because the signal voltages produced by these transmissions are cancelled out in a balancing circuit, indicated generally as 74 in FIG. 1.

The cancellation of the effects of spurious RF transmissions falling within the frequency range of the tuned antenna circuit can best be understood by referring to FIGS. 3 and 4 and the oscillator schematic shown in FIG. 1. The antenna circuits shown in FIGS. 3 and 4 are identical with the exception of the rectifier crystals 76 and 78 which are connected with reversed polarity with respect to each other. Any spurious transmission falling within the spectrum sensitivity of tuned dual antenna system will produce a voltage across each antenna tank circuit. The RF voltages developed in the tank circuits are detected by crystals 76 and 78 and produce the demodulated signal corresponding to the rectified RF. Since the crystal rectifiers 76 and 78 are connected 'with reversed polarities, as shown in FIGS. 3 and 4, the rectified signal voltages from the crystals will have corresponding reversed polarities. The RF from antenna assembly 4 is by-passed by capacitor 68 and the rectified and filtered signal appears across potentiometer 50. Similarly, the RF from antenna assembly 6 is by-passed by capacitor 68a and the rectified and filtered signal appear across potentiometer 52.

The rectified signal voltages from the two antenna assemblies are fed through the antenna feed lines 38 and 40 and chokes 42 and 44 to the balancing circuit comprising resistors 46 and 48, potentiometers 50 and 52 and resistors 54 and 56 as shown in FIG. 1. Potentiometers 50 and 52 are initially adjusted to compensate for any inherent imbalance in the circuit so that the output from the balancing circuit will be Zero when equal but opposite polarity signals are applied to the balancing circuit. If the antenna feed lines 38 and 40 are unequal in length, the balancing circuit 74 can be further adjusted to compensate for the additional signal attenuation produced by the longer feed line. In this situation a zero output would result from opposite polarity signals impressed on the balancing circuit which differed in magnitude by an amount equal to the additional signal attenuation of the longer feed line. In either case, however, it should be noted that the combination of the reversed polarity crystals 76 and 78 and the balancing circuit 74 provides a means for cancelling the effects of internally created transients from the oscillator 2. If the antenna feed lines are equal in length, the transients will produce equal but opposite polarity signals which are impressed on the balancing circuit and thereby cancelled. Similarly, if the antenna feed lines are unequal in length, the signals impressed on the balancing circuit will be opposite in polarity, but will difler in magnitude. However, the initial adjustment of potentiometers 50 and 52 will compensate for this inequality and the output will remain at zero. The output of the balancing circuit is taken from the junction of resistors 54 and 56 and represents the ditferential combination of the signals applied to the balancing circuit.

As mentioned previously, the spurious RF transmissions which fall within the spectrum sensitivity of the antenna systems will produce RF voltages across each antenna tank circuit. However, since the rectified signal voltages from the spurious transmissions will have equal but opposite polarities as a result of the reversed polarity crystals 76 and 78, the signals will be cancelled out in the balancing circuit 74 thereby producing a zero output to the moving object detection system.

The cancellation of the spurious RF transmissions is possible only because these transmissions normally strike both antenna assemblies substantially with the same modulation envelope thus creating equal but opposite polarity detected voltages which can be cancelled in the balancing circuit. A similar cancellation of signals could theoretically occur in the unusual situation created by the motion of the object to be detected along a line which is equidistant from both antenna assemblies. In this exceptional situation equal returned signals would be intercepted simultaneously by both antenna assemblies thereby causing equal but opposite polarity signals to be produced by the reversed polarity crystals '76 and 78. These signals would be cancelled in the balancing circuit in the same manner as signals produced by spurious transmissions falling within the bandpass of the tuned antenna. However, this is such an unusual situation and of such exacting symmetry that the probability of its occurring in a normal detection environment is non-existent.

Although a moving object within the detection field will also reflect radio frequency energy to both antennas, the returned energy received by one antenna will be different in phase and time with respect to the energy returned to the other antenna. Since the returned energy is different in phase and time, the rectified signals from the antenna assemblies 4 and 6 will not be equal and opposite in polarity. Therefore, the signal addition in the balancing circuit produces a resultant voltage whose amplitude and phase varies in accordance with the motion of the object through the standing wave pattern.

The combined or resultant signal is taken from the junction or resistors 54 and 56 on line 80. Line 80 emerges from the oscillator assembly shield 82 through a feed-through by-pass capacitor 84 and is capacitively coupled to the input of the audio amplifier 10 by capacitor 8. The capacitive coupling of the balancing circuit to the audio amplifier prevents the detection system from responding to the steady state voltage produced by the rectification of the oscillator RF energy while coupling the fluctuations in signal voltage representing the motion of the object through either the standing wave pattern created by placing the antennas Within an enclosed space or the radiation field established by positioning the antennas in a non-confined substantially non-reflecting environment. This motion induced signal fluctuation varies in the order of 1 to 10 cycles per second and hence the coupling and amplifier response includes the sub-audio range.

The sub-audio signal representing in amplitude and phase the presence of a moving object Within the detection area is amplified by the audio amplifier 10 and applied to a low pass filter 12 having a bandpass of 0.1 to 10 cycles per second. The signals within this frequency range are applied to the indicator 14 to provide a visual, aural or electrical indication of the presence of a moving object within the standing Wave pattern established by the antenna assemblies. The indicator 14 obviously may be connected to any type of alarm circuit or other signalling means placed within the detection environment or located at a position remote from the protected area. The circuits for the audio amplifier, low pass filter and indicator have not been shown because they are well known in the art and many variations therein could be employed to accomplish the results described above.

Although the dual antenna system has been shown as the preferred embodiment, it is apparent that all of the features previously described are obtainable with a single antenna assembly with the one exception of signal cancellation in the balancing circuit. It is also obvious that numerous other circuits may be employed to process the data represented by the output of the combining circuit 74, and therefore it is not intended to confine the invention to the precise form shown herein, but rather to be limited in scope only by the appended claims.

Having thus described and disclosed the preferred embodiments of my invention, what I now claim as new and desire to secure by Letters Patent of the United States 1. In an electromagnetic object detection device the combination of: means including a radio energy transmitter for producing a standing wave pattern between said device and a reflecting surface, said pattern containing spaced nodes and anti-nodes of radio frequency energy; means for detecting changes in the locations of said nodes and anti-nodes caused by a change of position of a reflecting object in the said field of said pattern said means including a pair of spaced antennas, separate means for quantitatively sensing impedance changes at each said antenna with the output of one said sensing means being inverted with respect to the other, means for differentially combining the outputs of said sensing means, and said antennas being spaced apart by a distance greater than the distance at which said antennas respond substantially identically to changes in said standing wave pattern.

2. The object detection device defined in claim 1 further characterized by said antennas both also serving to transmit said radio energy and being located with respect to each other so that the transmissions of each contribute significantly to the standing wave pattern surrounding the other.

3. In an elecrom-agnetic object detection system, the combination comprising: a continuous wave signal generator; a pair of antennas for radiating and receiving energy; means for coupling said signal generator to said antennas; opposite polarity detector means coupled to said antennas to produce opposite polarity detected signals (from said antennas; means coupled to said detector means for differentially combining the detected signals from said detector means; and an indicator coupled to said combining means through a capacitance whereby said indicator is responsive only to signals from said combining means representing the combination of varying signals imposed on said detector means.

4. In an electromagnetic object detection system, the combination comprising: a continuous wave signal generator; a first antenna and a second antenna for radiating and receiving energy; means for coupling said signal generator to a first antenna feed line and a second antenna feed line, said feed lines terminating in a first and a second parallel resonant circuit respectively, said circuits being resonant at the frequency of said signal generator; a first capacitance coupling said first resonant circuit to said first antenna, a second capacitance coupling said second resonant circuit to said second antenna; a first and a second electrical means for varying the resonance of said first and sec-ond parallel resonant circuits, respectively, whereby the energy coupled from said signal generator to said antennas through said feed lines, resonant circuits and capacitances is varied; a first and a second detector means coupled to said first and second parallel resonant circuits, respectively, said detector means being oppositely poled to produce opposite polarity detected signals from said circuits; means coupled to said detector means for differentially combining the detected signals from said first and second detector means whereby the opposite polarity detected signals produced by the effect of spurious energy upon said opposite polarity detector means will be cancelled; and an indicator coupled to said signal combining means through a capacitance whereby said indicator is responsive only to signals from said combining means representing the combination of varying signals imposed on said detector means.

5. In an electromagnetic object detection system, the combination comprising: a continuous wave electron tube signal generator having a generally U-shaped plate line inductance; a generally U-shaped coupling inductance disposed in a plane parallel to the plane of said plate line inductance whereby the continuous wave of said signal generator is coupled by mutual inductance from said plate line inductance to said coupling inductance; a first antenna feed line coupled to one end of said coupling inductance through a first capacitance and a second antenna feed line coupled to the other end of said coupling inductance through a second capacitance, said feed lines terminating in a first and a second parallel resonant circuit respectively, said circuits being resonant at the frequency of said signal generator; at first antenna and a second antenna for radiating and receiving energy; a third capacitance coupling said first resonant circuit to said first antenna; a fourth capacitance coupling said second resonant circuit to said second antenna; a first and a second electrical means for varying the resonance of said first and second parallel resonant circuits, respectively, whereby the energy coupled from said signal generator to said antennas through said feed lines, resonant circuits and capacitances is varied; a first and a second detector means coupled to said first and second parallel resonant circuits, respectively, said detector means being oppositely poled to produce opposite polarity detected signals from said circuits; means coupled to said detector means for combining differentially the detected signals from said first and second detector means whereby the opposite polarity detected signals produced by the effect of spurious energy upon said opposite polarity detector means will be cancelled; and an indicator coupled to said signal combining means through a capacitance whereby said indicator is responsive only to signals representing the combination of varying signals imposed on said detector means.

6. In an electromagnetic object detection system, the combination comprising: a continuous wave signal generator; an antenna for radiating and receiving energy, said antenna having a truncated hemispherical base and a substantially quarter wave vertical radiator mounted on the truncated portion of said base and insulated therefrom; means for coupling said signal generator to an antenna feed line, said feed line terminating in a parallel resonant circuit resonant at the frequency of said signal generator and comprising an inductance and a capacitance mounted within the hemispherical base of said antenna; a capacitance mounted within said base for coupling the resonant circuit to said radiator; electrical means mounted within said base for varying the resonance of said parallel resonant circuit whereby the energy coupled from said signal generator to said an tenna through said feed line, parallel resonant circuit and capacitance is varied; a detector coupled to said parallel resonant circuit in parallel connection thereto and responsive to variations therein produced by the motion of said object, said detector including in series connection a rectifier mounted within said hemispherical base and a filter circuit, said filter circuit having in parallel connection a resistance and a capacitance; and an indicator coupled to said filter circuit through a capacitance whereby said indicator is responsive only to varying signals imposed on said detector.

7. In an electromagnetic object detection system, the combination comprising: a continuous wave signal generator; a pair of antennas for radiating and receiving energy; means for coupling said signal generator to said antennas; opposite polarity detector means coupled to said antennas to produce opposite polarity detected signals from said antennas; a first branch circuit connected to one of said detector means having a first resistance and fourth resistance-s having the other ends thereof connected to the other of said detector means having a second resistance and a second potentiometer; a third resistance having one end thereof connected to the movable contact of said first potentiometer and a fourth resistance having one end thereof connected to the movable contact of said second potentiometer, said third and fourth resistances having the other ends thereof connected together whereby the output taken from the junction of said third and fourth resistances represents the differential combination of the signals impressed on said first and second branches; and an indicator coupled to the junction of said third and fourth resistances through a capacitance whereby said indicator responds only to signals representing the combination of varying signals impressed on said detector means.

8. In an electromagnetic object detection system, the combination comprising: a continuous wave signal generator; an antenna for radiating and receiving energy having a base and a substantially quarter wave vertical radiator mounted thereon and insulated therefrom; means for coupling said signal generator to an antenna feed line, said feed line terminating in a parallel resonant circuit resonant at the frequency of said signal generator and comprising an inductance and a capacitance mounted within said antenna base; means for coupling said resonant circuit to said radiator; detector means coupled to said parallel resonant circuit, said means including a rectifier mounted within said antenna base; and an in dicator coupled to said detector means through a capacitance whereby said indicator is responsive only to varying signals impressed on said detector means.

9. In an electromagnetic object detection system, the combination comprising: a continuous wave signal generator; an antenna for radiating and receiving energy having a base and a substantially quarter wave vertical radiator mounted thereon and insulated therefrom, means for coupling said signal generator to an antenna feed line, said feed line terminating in a parallel resonant circuit resonant at the frequency of said signal generator and comprising an inductance and a capacitance mounted within said antenna base; a capacitance mounted within said base for coupling said resonant circuit to said radiator; electrical means mounted within said antenna base for varying the resonance of said parallel resonant circuit; detector means coupled to said parallel resonant circuit, said means including a rectifier mounted within said antenna base; and an indicator coupled to said detector means through a capacitance whereby said indicator is responsive only to varying signals impressed on said detector means.

10. The electromagnetic object detection device of claim 1 wherein said means for differentially combining the outputs of said sensing means comprises a resistance network.

References Cited by the Examiner UNITED STATES PATENTS 2,247,246 6/ 1941 Lindsay et al. 340258 2,424,677 7/ 1947 Brownlee 34025 8 2,490,238 12/1949 Simons 340258 2,656,527 10/ 1953 Tillman 340-258 2,660,718 11/ 1953 Summerhayes et al. 340258 2,808,278 10/ 1957 Snyder 343900 2,826,753 3/ 1958 Chapin 340--258 2,956,269 10/ 1960 Schmidt 340258 3,005,191 10/ 1961 Schmidt 340-258 3,022,499 2/1962 Ripepi 340258 CHESTER L. JUSTUS, Primary Examiner.

KATHLEEN CLAFFY, Examiner. P. M. HINDE'RSTEIN, Assistant Examiner. 

1. IN AN ELECTROMAGNETIC OBJECT DETECTION DEVICE THE COMBINATION OF: MEANS INCLUDING A RADIO ENERGY TRANSMITTER FOR PRODUCING A STANDING WAVE PATTERN BETWEEN SAID DEVICE AND A REFLECTING SURFACE, SAID PATTERN CONTAINING SPACED NODES AND ANTI-NODES OF RADIO FREQUENCY ENERGY; MEANS FOR DETECTING CHANGES IN THE LOCATIONS OF SAID NODES AND ANTI-NODES CAUSED BY A CHANGE OF POSITION OF A REFLECTING OBJECT IN THE SAID FIELD OF SAID PATTERN SAID MEANS INCLUDING A PAIR OF SPACED ANTENNAS, SEPARATE MEANS FOR QUANTITATIVELY SENSING IMPEDANCE CHANGES AT EACH SAID ANTENNA WITH THE OUTPUT OF ONE SAID SENSING MEANS BEING INVERTED WITH RESPECT TO THE OTHER, MEANS FOR DIFFERENTIALLY COMBINING THE OUTPUTS OF SAID SENSING MEANS, AND SAID ANTENNAS BEING SPACED APART BY A DISTANCE GREATER THAN THE DISTANCE AT WHICH SAID ANTENNAS RESPOND SUBSTANTIALLY IDENTICALLY TO CHANGES IN SAID STANDING WAVE PATTERN. 